Compression?

[LONG-ROD]

I was wanting to know about how much compression I sould have. I have a stock cylinder that was decked, said took barely any off, stock gasket, duncan power head with 19cc dome. any ideas? My tester said 170, I run 110 and runs fine 102 and detination occurs, is time for a new tester?

[Kirt13]

I have heard from a few old timer pro builders that you don't need race gas until you get up to about 190-200 compression.
I would definitely test the compression with a new or proven compression tester.
Kirt

[LONG-ROD]

Well it defintly needs race fuel, it was built for race gas so it should.

[Honda 250r 001]

170 is fine to run on pump gas. On my 250r i am running 185 comp and it has ran like that for a long time. somethings wrong. you dont mil the head, and run a smaller dome and get 10 pounts over stock compression.

[Kirt13]

Originally posted by LONG-ROD
Well it defintly needs race fuel, it was built for race gas so it should.
I think maybe you don't understand the reason for an engine to need a higher octane fuel (race gas).
An engine isn't built for a higher octane.
You need a higher octane depending on the compression level.
Any body can put on a smaller dome to raise the compression and this would cause them to need to run a higher octane fuel.
"Race gas" does not mean it's going to make your engine faster. It just detonates at a different rate. A higher compression engine needs this detonation rate to run properly.
Buying "race gas" for an average compression engine is just wasting your money.
Kirt

[kiesta00]

Whats your porting like? top ended? Whats your ignition timing set at? There are some other variables that will contribute to detonation other than cranking compression.

Are you kicking the crap out of it with the throttle wide open? Try a different compression tester.

[LONG-ROD]

Ok, 90% of you need to read the post agin, I have not milled the head, I decked the cylinder, big diffrence. I no exactly how race gas works, and detination. I said my motor detinates on pump gas, and more. Yes you do build a engin to run on race fuel. I have a 310 sleeve at around 255 cc so I have a really thick sleeve that heat does not disapate heat through it easy, so I built this motor to run 195 compression or a little more so It would run on VP C12. THis will help keep the temps down in the combustion chamber. I am pretty sure my tester is reading wrong. I just pretty much wanted some people who have built simler motors to let me know where there compression is. Timeing has been left alone, and port work is minimal. Lauran duncan rechambered them for the sleeve then said gave me a mild but not full port job, he new I was shooting for mid rang pull.

[86 Quad R]

you running the thick OEM head gasket? how far down in the bore is the piston @ TDC? how deep is the squish band cut on the cylinder head? how much overall squish do you have?

fyi: i've seen the piston to cylinder deck distance vary from .075(1.905mm) to as much as .120(3.0mm) and as you can see drastically affects the compression.

i recently built two engines that were both about a 3rd overbore and the porting was about the same. same piston, same head, head gasket, carb and reed setup. one was about 190# and the other was 170# after breakin. the only difference was in deck height.

in a nut shell............ its possible that your gauge is right.

[LONG-ROD]

I will get some time next week I will pull the head off and check these things, I will also check the squish, it would be good to no. oh and I am running the 3 peice gasket.

[derby]

You would be better off finding the volume in the head while the piston is at top dead center. I always used vegetable oil with a syringe to find mine. You can put the piston at tdc with the head on, then start filling with vegetable oil until it reaches the bottom of the treads on the spark plug hole. You just have to keep track of how many cc's you add.

[LONG-ROD]

never heard of that one, I have bigger problems now

[derby]

I am not aware of a better way to get your exact volume than that.

[machwon]

Here's a link to a chart which is a good starting point for octane requirements and full stroke compression ratio.

http://www.msnusers.com/RBRacingTwoSTrokePorting/Documents/Fuel%20Chart%2Ejpg

I always measure the trapped volume of the head as Derby mentioned and I never go by the static cranking psi. There are too many variables when it comes to base/head gaskets, cylinder decking, head milling and porting for someone to just quote you a cranking psi and the required octane. Other factors also play a part, but the compression ratio and that chart will get it real close.